Generally, a generator such as a magneto generator is mounted to an engine for driving a vehicle, and a battery that supplies electric power to various electrical components is charged by an output of the generator. Electric units such as a control unit of various electrical components or a voltage regulator are connected to both ends of the battery. Many of the electric units connected to the battery require passage of a large current.
In recent years, a rotating electric machine that operates as a starter motor at a start of an engine and operates as a battery charging generator after the start of the engine has been developed and in practical use. The rotating electric machine used for this purpose is referred to as a starter generator that serves as both a starter motor and a battery charging generator. In order to operate the starter generator, a control unit that controls energization from the battery to the starter generator and energization from the starter generator to the battery needs to be connected to the battery. A large current of about 100 A is passed through the control unit in the energization from the battery to the starter generator to operate the starter generator as the starter motor.
A starter generator is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2000-209891. The starter generator disclosed in Japanese Patent Application Laid-Open Publication No. 2000-209891 comprises a rotor having a magnetic field and mounted to a crankshaft of an engine, and a stator having a polyphase armature coil wound around an iron core having magnetic pole portions facing magnetic poles of the rotor, and is operated as a brushless motor at a start of the engine and operated as a magneto generator used for charging a battery after the start of the engine.
As disclosed in Japanese Patent Application Laid-Open Publication No. 2000-209891, a control unit used for operating the starter generator is comprised of a driver and a controller that controls the driver. The driver comprises a full-wave rectifier circuit that rectifies an AC output supplied from armature coils and supplies the DC output to a battery in operation of the starter generator as the generator, and an inverter-type switch circuit that supplies a drive current from the battery to the starter generator in operation of the starter generator as the motor.
Generally, an on-vehicle electric unit connected to a battery is supported in a position away from the battery, and the electric unit and the battery are connected by a wire. FIG. 8 shows an example of a vehicle including a starter generator. The shown vehicle 1 is an ATV (an All Terrain Vehicle, so-called a buggy), and comprises a vehicle body 3 including an engine 2, a pair of front wheels 5 operated by a steering handle 4, and a pair of rear wheels 6, and the front wheels 5 and the rear wheels 6 are driven by the engine 2. A reference numeral 7 denotes a starter generator (SG), which is comprised of a rotor having a magnetic field and mounted to a crankshaft of the engine 2, and a stator having three-phase armature coils wound around an iron core and secured to a case of the engine.
In the shown example, a battery 8′ and a control unit 9′ are included in rear and front portions of the vehicle body 3, and a positive DC terminal and a negative DC terminal of the control unit 9′ and a positive terminal and a negative terminal of the battery 8′ are connected by wires 10 and 11, respectively. Three-phase AC terminals of the control unit 9′ are connected to terminals of three-phase armature coils of a starter generator by three wires 12u to 12w. 
A driver provided in the control unit 9′ comprises a full-wave rectifier circuit that rectifies an AC output of the armature coils of the starter generator, and thus if the battery 8′ is accidentally reversely connected, the battery 8′ is short-circuited by diodes that constitute the full-wave rectifier circuit. When the battery 8′ is thus short-circuited, a large short-circuit current is passed from the battery through the full-wave rectifier circuit, which destroys rectifier elements that constitute the rectifier circuit in an instant.
In such a driver of the control unit, as disclosed in Japanese Patent Application Laid-Open Publication No. 2000-209891, MOSFETs are often used as switch elements that constitute an inverter-type switch circuit, and parasitic diodes formed between drain and source thereof constitute the full-wave rectifier circuit. In this case, if the battery is reversely connected, the MOSFETs themselves are destroyed. In order to prevent such an accident, as disclosed in Japanese Patent Application Laid-Open Publication No. 2001-69796, providing a protection device is proposed comprising a relay having a contact inserted between a battery and a driver, and a control circuit that controls to excite the relay to close the contact only when the battery is connected in a correct direction.
However, if the battery 8′ and the electric unit (the control unit 9′ in the above example) are connected by the wires as in the conventional vehicle, vibration transferred from the vehicle body causes large vibration of the wires to apply a large force to connecting portions between the wires 10 and 11 and the battery 8′ and connecting portions between the wires 10 and 11 and the control unit 9′, which may cause detachment of the wires 10 and 11 from the battery 8′ and/or the control unit 9′ or loosen the connecting portions. Particularly for a vehicle such as an ATV for driving on rough ground, large vibration is transferred from the vehicle, and thus it is not preferable that the battery and the control unit are connected by the long wires as shown in FIG. 8. When wires 10 and 11 having large diameters are used for passing a particularly large current, the wires have large masses, thereby prominently causing the above described problem.
Also, when the battery and the electric unit are connected by the wires, routing of the wires is troublesome, thereby increasing the number of steps of assembling the electric unit to the vehicle to increase costs. Thus, the number of places connected by the wires is desirably as small as possible.
Further, when the battery and the electric unit are connected by the wires, electric resistance of wiring between the battery and the electric unit increases to increase losses in the necessity of passing a large current from the battery to the electric unit.
As disclosed in Japanese Patent Application Laid-Open Publication No. 2001-69796, when the protection device is provided in the control unit to guard against accidental reverse connection of the battery, the size and also the cost of the control unit 9 increase.
In the above description, the case where the electric unit connected to both ends of the battery is the control unit of the starter generator has been described by way of example, but the same problem occurs when the electric unit connected to the battery is any other unit such as a voltage regulator.